What you really sign up for when you use social media

Facebook, Twitter, Instagram. Odds are you are a current user of at least one of these social sites.

But do you really know what you have signed up for to use their services?

Unless you are an attorney or a privacy advocate, you probably do what most people do and quickly scroll through terms of service agreements and privacy policies ignoring the fine print to get registered for a new account.

Social Media apps Linkedin Twitter Facebook
Brent Lewin | Bloomberg | Getty Images

In fact, half of online Americans don't even know what a privacy policy is, according to a Pew Research Report published last year.

While privacy policies were originally intended to be a protection mechanism for consumers, they have turned into "ownership policies," said Nico Sell, CEO of the Wickr Foundation, the non-profit arm of a company that bills itself as the world's most trusted messenger site. And now corporations have way more rights to users' data than most people realize, she said.

Read MoreThe next thing you'll pay for: Your online privacy

Whose content is it?

Most reputable social media companies like Facebook, Twitter and LinkedIn require users to agree to a terms of service agreement before registering an account.

While these contracts generally don't give ownership of published content to the social media companies, the agreement does usually secure the companies a broad license to use anything users post to their platforms.

For example, Instagram's terms of use states: "Instagram does not claim ownership of any Content that you post on or through the Service. Instead, you hereby grant to Instagram a non-exclusive, fully paid and royalty-free, transferable, sub-licensable, worldwide license to use the Content that you post on or through the Service."

Read MorePrivacy compromised? Might as well monetize

Most social sites have similar language. This enables the sites to have an expansive right to use all posted or shared content without being liable to you, Sell said.

"I think people would be shocked to find out that almost every single privacy policy is the same, whether it's WhatsApp, Snapchat, Twitter, Facebook you are granting a free transferable worldwide license for what you input into their free service. And so what does that mean? To us, that means things you share with those services, you are giving up or deciding ownership and control of your personal messages and photos with a large corporation," she said.

While Facebook does not currently give third party applications or ad networks the right to use a user's name or picture in ads, it does reserve the right to use a user's name and profile picture in social ads on its own platform.

For example, if a user "likes" a brand on Facebook, the person's name and picture may show up alongside that brand's advertising message in a friend's newsfeed, but only if the user has opted to share their "likes" with friends.

You are giving up rights to your big data

Social media giants not only have a license to use content that you post, but they are also constantly collecting data on you that you may not realize you are sharing.

For example, Facebook collects information from all devices you have installed it on or access its services from. So depending on the permissions you have granted Facebook, it can collect things like your device location via GPS, Bluetooth or Wi-Fi signals. It can also collect information such as the name of your mobile provider, Internet service provider and your language and time zone, according to its data policy.

Twitter collects similar metadata and can track users' whereabouts even if their location services is disabled. The company may do this by using publicly broadcast information from the user's wireless access point, like a MAC Address, to identify their approximate location, according to the company's website.

And both sites collect data about users from third party websites that use the companies' services. So for example, if you visit a website that has the "like" button implemented on it or a site that uses Facebook's advertising service, the social media company collects data about your visit to that site.

"When you sign up for those terms of service and you click on the "I agree" button, you are also giving away rights to your big data," said Brad Frazer, an Internet IP and IT lawyer at Hawley Troxell.

"You are essentially just a commodity, you are big data generating commodity.That's really what you are, that is how social media sees you, with a big dollar sign on your forehead generating big data they can sell. And that is something else you have given up by contract."

Read MoreUS tech giants face fresh fight with Europe over privacy

Your data may not be deleted when you think it is

If you delete your account, there's a good chance that not everything disappears as quickly as you might want.

"We store data for as long as it is necessary to provide products and services to you and others," Facebook's data policy states.

According to Facebook's website, it could take 90 days to delete all of the things a user has posted, including pictures, status updates and other data stored in its backup system. However, once an account is deleted, this information will no longer be available for other users to view.

Information others have shared about you will not be deleted, however, as it is not part of your account.

On Instagram, you can't really delete your account at all. However, you can deactivate it.

According to Instagram's terms of service, if you choose to deactivate your account your photos, comments, likes, friendships and other data won't be available through your account. However that material and data may still exist and appear within the service. So for example, your profile and images could no longer be viewed from your page, but if someone reposted a picture that you originally published, it would still exist on the platform.

Your posts can end up anywhere

With all the social media faux pas these days, it might seem obvious that everything you do online can end up just about anywhere, regardless of how private you intended it to be.

While privacy settings on sites like Instagram, Facebook or Twitter may limit the reach of your original post, there's no way to really rein in how others on the social site may use your publicly shared content.

For example, if you post a picture to your profile and have privacy settings set to only share with your friends, there's still the chance that a friend could always take that post and share it elsewhere. So while Facebook is using its license to your content to distribute as you intended doesn't mean everyone else will respect that.

"Even though you are aware you are granting certain rights to Facebook, Twitter or Pinterest or Vine or whoever, you may not necessarily [be] granting those rights downstream to the other users of the site," Frazer said. "You still need to be vigilant to see how other people are using your content. Just because I granted a license to Facebook, doesn't mean that I granted it to some kid I knew in high school to take my photo and put it on a thousand T-shirts," he added.

"Think about the consequences of your posting because it really could end up anywhere."